Process for manufacturing aluminum alloys and aluminium castings

ABSTRACT

A process for manufacturing parts made of aluminum cast or aluminum alloy cast in a mold, which includes after casting, removing from the mold the part which constitutes the cast preform while still hot, and placing it between two halves of a die that defines a cavity having dimensions that are slightly less than those of the mold, forcing said two halves together in order to exert a combined in-depth impact forging and surface hardening effect on the casting placed between the two halves. The invention further includes the step of transferring the cast preform prior to the impact forging step at a temperature of about 400 to 500° C. to a furnace that ensures solution heat treatment for the aluminum alloy at its homogenization temperature.

[0001] The invention relates to the technical field of manufacturing aluminium alloy castings, and aluminium casting obtained using foundry, forging and similar processes.

[0002] Patent EP 119 365 discloses a specific process referred to as COBAPRESS which combines foundry and forging techniques for parts made of aluminium or aluminium alloy in particular.

[0003] This process involves casting the aluminium or aluminium alloy in a mould and, after casting, removing the still hot part from the mould at a temperature of the order of 400 to 500° C. and placing it between two halves of a die that defines a cavity having dimensions that are slightly less than those of the mould, the two halves being forced together in order to exert a combined in-depth impact forging and surface hardening effect.

[0004] This process referred to as COBAPRESS is widely used in numerous applications, security components such as chassis and engine parts, control arm, knuckle, engine bracket and other typesand for bicycle parts such as pedal cranks.

[0005] After carrying out the above-mentioned process and the forging operation in particular, the parts are allowed to cool at room temperature.

[0006] A trimming and straightening operation must then be performed in certain cases if the part is not properly formed. This may be followed by heat treatment at a temperature of roughly 540° C. for the solution heat treatment. The part is then allowed to coal down to room temperature with a quench operation.

[0007] The successive phases involved in the casting/forging process have drawbacks due to the additional lime that is require red, the lack of control of the residual time after the forging. phase and cooling to room temperature which corresponds to storing the parts as they are for an indeterminate time before final solution heat treatment.

[0008] The object sought after by the invention was to optimise. the process described in above-mentioned Patent EP 119 365 in order to reduce the time needed to produce parts, taking all the treatments into consideration, and to eliminate the final heat treatment.

[0009] Various research work was carried out essentially after the forging operation, in order to limit the non-productive time. Experiments did not produce continuous satisfactory results over a long period because of the excessive difficulty in controlling the time between the final forging phase of the process and the quench hardening after heat treatment.

[0010] The original feature of the invention involves introducing an intermediate step between the foundry phase in which the cast preform is obtained and the forging phase, with the foundry phase involving casting the liquid aluminium at a temperature of 750° C. in a mould that is heated and kept at a temperature of roughly 400° C., the aluminium cast in the mould in the liquid state tending to cool until it reaches the temperature of the mould of roughly 400° C., thereby. producing a preform that is then transferred to a furnace preheated to a temperature of roughly 540° C. in order to ensure solution heat treatment of said cast preform at said temperature of 540° C.,

[0011] transferring the treated preform and heating it to its natural solution heat temperature in a die that defines a cavity having dimensions that are slightly less than those of the mould, the two halves of the die being forced together in order to exert a forging/die forging force to obtain the final rough forging,

[0012] cooling the rough forging thus obtained at room temperature or accelerated cooling or cooling by quench.

[0013] According to another aspect, the process is distinctive in that it employs the following operations:

[0014] introducing an intermediate step between the foundry phase in which the cast preform is obtained and the forging phase, with the foundry phase involving casting the liquid aluminium at a temperature of 750° C. in a mould that is heated and kept at a temperature of roughly 400° C., the aluminium cast in the mould in the liquid state tending to cool until it reaches the temperature of the mould of roughly 400° C., thereby producing a preform that is then transferred to a furnace preheated to a temperature of roughly 540° C. in order to ensure solution heat treatment of said cast preform at said temperature of 540° C.,

[0015] transferring the treated part heated to approximately 540° C. between two halves of a die defining a cavity having dimensions that are slightly less than those of the mould, the two halves being forced together in order to exert an impact forging force;

[0016] allowing the part to cool at room temperature, or accelerated cooling, or cooling by quench.

[0017] These aspects and others will become apparent from the remainder of the description.

[0018] In order to make the invention more readily understood, we have illustrated, in the form of graphs, the various operating phases and heating phases' corresponding to use of the COBAPRESS process described in Patent EP 119 365 (FIG. 1) then those involved in accordance with this invention in a first version of the process that includes an intermediate phase between casting and forging the part (FIG. 2), then a second version of the process according to the invention that involves other operating phases after forging the part.

[0019] More specifically, the process for manufacturing parts made of cast alloy, especially aluminium, is distinctive in that it involves the following operations:

[0020] casting at approximatively 750° C. the preform made of alloy, especially aluminium and aluminium alloy, in a mould at a temperature of the order of 400 to 500° C.;

[0021] immediately transferring the cast preform thus obtained at the temperature at which it leaves the mould to a furnace that ensures solution heat treatment of said cast preform at a temperature of the order of 540° C.;

[0022] transferring the treated part heated to approximately 540° C. between two halves of a die that defines a cavity having dimensions that are slightly less than those of the mould, the two halves being forced together in order to exert an impact forging force;

[0023] allowing the part to cool at room temperature, or accelerated cooling or cooling by quench, this being obtained without the need to perform any final heat treatment as was the case according to the prior art.

[0024] Regarding the FIG. 2, the final operation may be possible to add consisting in a natural ageing, depending on the desired mechanical properties.

[0025] According to the invention, the furnace used after first casting the part is a tunnel furnace. The time to heat it to substantially 540° C. which corresponds to the temperature of the solution heat treatment at varies from 40 minutes to 2 hours and is advantageously 1 hour.

[0026] As it leaves the tunnel furnace, with the contact with the ambient temperature and this tends to reduce its temperature as it is transferred to the forging station. This transfer is performed in any appropriate manner using appropriate handling means. The impact forging temperature remains approximately 530° C. and this makes it possible to ensure that the part has all the required mechanical strength properties.

[0027] The process thus described in accordance with the invention makes it possible to eliminate the heat treatment after the forging operation and straightening that were initially needed to prevent and correct any deformation of the part using the process according to the prior art.

[0028] According to the invention, the intermediate treatment operation in the tunnel furnace for a period of roughly 1 hour makes it possible to eliminate the operations that were performed using the previous technique after the casting/forging operations, and which took place over a period of 6 to 8 hours, whilst nevertheless obtaining properties that are substantially equal to those obtained using the prior art.

[0029] Experiments were performed and produced the following results on an AS7GO3 or A356 alloy (US grade) or LM25 (UK grade): Conventional EP 119 365 process following casting, Process forging and heat according to treatment the invention operations Proof stress Rp 0.2 Average 210 Average 212 mpa mpa Material strength Average 290 Average 298 mpa mpa Yield point Average 8.8% Average 9.5%

[0030] The process according to the invention thus makes it possible to optimise the COBAPRESS process described in Patent EP 119 365.

[0031] Another development of the invention involves performing other operating phases if there is a requirement to enhance the performance of the invention further.

[0032] Thus, according to an additional variant, the three operations (casting, heat treatment in a tunnel furnace at 540° C. and impact forging) are performed as indicated earlier and are then supplemented by a fourth operating phase after impact forging which involves performing an additional solution heat treatment operation on the part in another tunnel furnace at approximately 540° C. for a period of 40 minutes to 2 hours and preferably 1 hour. This fourth operation is followed by a fifth quench hardening with cold water. In a final operation, the parts undergo a natural ageing.

[0033] This process makes it possible to enhance the results and homogenise the treated parts.

[0034]FIGS. 1, 2 and 3 show the various temperature stages. The period x (FIG. 1) corresponds to the dwell time of the part obtained at room temperature in accordance with Patent EP 119 365 before the subsequent quench hardening operation. Period y (FIGS. 2 and 3) corresponds to the solution heat treatment in the tunnel furnace. The process perfected in accordance with the invention, in both its embodiments, allows a substantial reduction in production time and manufacturing costs of the order of approximately 8 to 10% whilst nevertheless obtaining the same part properties compared with the COBAPRESS process described in EP 119 365 

1- Process for manufacturing parts made of cast alloy, especially aluminum cast or aluminium alloy cast, in a mould and which comprise, after casting, removing from the mould the part which constitutes the cast preform which is still hot and placing it between two halves of a die that defines a cavity having dimensions that are slightly less than those of the mould, forcing the two halves together in order to exert a combined in-depth impact forging and surface hardening effect on said casting placed between the two halves, Wherein the original feature of the invention involves introducing an intermediate step between the foundry phase in which the cast preform is obtained and the forging phase, with the foundry phase. involving casting the liquid aluminium at a temperature of 750° C. in a mould that is heated and kept at a temperature of roughly 400° C., the aluminium cast in the mould in the liquid state tending to cool until it reaches the temperature of the mould of roughly 400° C., thereby producing a preform that is then transferred to a furnace preheated to a temperature of roughly 540° C. in order to ensure solution heat treatment of said cast preform at said temperature of 540° C., transferring the treated preform heated to a die that defines a cavity having dimensions that are slightly less than those of the mould, the two halves of the die being forced together in order to exert an impact forging/die forging force in order to obtain the finished rough forging, cooling the forging casting thus obtained at room temperature or accelerated cooling or cooling by quench hardening. 2- Process as claimed in claim 1, characterised in that high temperature homogenisation time correspond to the heating time in order to obtain the same part criteria varies and takes approximatively one hour. 3- Process as claimed in claim 1, characterised in that the three operations of casting, heat treatment in a tunnel furnace at 540° C. and then impact forging are supplemented by a fourth operating phase after impact forging which involves carrying out an additional solution heat treatment operation on the part in another tunnel furnace at approximately 540° C. for a period of about 1 hour, and in that this fourth operation is followed by a fifth quench hardening operation. 4- Process as claimed in claim 1, characterised in that the parts obtained after the final operation and the second quench undergo natural ageing. 5- Process as claimed in claim 3, characterised in that the parts obtained after the final operation and the second quench undergo natural ageing. 